1. Field of the Invention
The invention relates to a method for dehydrating solid products containing moisture, in particular food products, according to which a hot gas stream is guided over the products while they are exposed to ultrasonic waves.
2. Description of the Related Art
Traditional methods for desiccating or dehydrating food products by a forced stream of hot air are relatively economical, but the elimination of the interior moisture takes a relatively long time. Moreover, high temperatures may damage the food, which in certain cases may change the color, the taste, the possibility of rehydration and the nutritional value.
Other methods may exclude these disadvantages, but certain methods, such as freeze-drying, are expensive, and others, such as drying by pulverization, are only application to liquids or pastry.
It is known that supplying vibrational energy may stimulate the dehydration.
For instance, U.S. Pat. No. 2,297,726 describes a method according to which atomized products are subjected to sound waves and subsequently to an evaporation.
U.S. Pat. No. 2,344,754 describes dehydration by a sonic or ultrasonic vibration in combination with high frequency electric currents.
U.S. Pat. No. 3,175,299 relates to the desiccation of sugar crystals by sound waves.
According to U.S. Pat. No. 3,592,395, deposits or fine powders are desiccated in an agitated fluidized bed. The agitation tends to reduce the size of the particles of the product, which diminishes the commercial value. Moreover, this agitation may damage delicate products.
Another application of sonic vibration for dehydrating is described in U.S. Pat. No. 3,641,680. The electrostatic removal of the moisture from a humid lye or from deposits is supported by vibration.
The combination of hot air and sound waves for dehydrating food products is already known from U.S. Pat. No. 4,334,366. The food products are introduced in a perforated rotating drum and heated by a pulsating burner providing sonic vibrations. The sonic energy, or more particularly the succession of compression-expansion cycles which is provokes, removes the moisture from the surface of the products. The hot air is necessary for removing, by convection, the moisture from the interior of the products. The efficiency of the dehydration increases with the temperature; therefore, the majority of such devices having a pulsating burner provide for an increased temperature, for example, of more than 1000.degree..
At this high temperature, the drying time is a few fractions of a second which makes it difficult to control the working conditions and renders the method unsuitable for dehydrating heat-sensitive food products. The application of lower temperatures, however, strongly diminishes the efficiency of the method.
The above-mentioned method causes sound leaks towards the environment above 140 dB which necessitates a considerable silencing. Moreover, the consistency of the food products is difficult to preserve.
According to U.S. Pat. No. 4,708,159, this method has been improved, but its application has been limited to solutions or deposits.
Consequently, it is known that sound vibration improves the efficiency of dehydration, but in most cases, particularly for desiccating food products, the improvement has proved to be insufficient to justify commercialization.